In optical switches that switch optical paths in optical transmission lines, micro-mirror elements (MEMS mirror elements) that reflect light output from an optical fiber to an optical path of any optical fiber via a rotating (swinging) mirror have been conventionally used (e.g., see Patent Document 1).
FIG. 24 is an overview plan view illustrating a related micro-mirror array.
In FIG. 24, a micro-mirror array 81 has a number of micro-mirror elements 82 arranged in one direction. A mirror 83 in each of the micro-mirror elements 82 is coupled to a mirror-side movable electrode (comb-like electrode) 85 via a mirror arm 84. The mirror 83 is supported by an inner frame 87 via a pair of torsion bars 86, 86 in the mirror arm 84, and is rotatable (swingable) around a rotation axis of the torsion bars 86, 86.
The inner frame 87 is disposed so as to cover the mirror 83 and the mirror side movable electrode 85, and has a mirror-side fixed electrode (comb-like electrode) 87a placed opposite the mirror-side movable electrode 85. By electrostatic force generated by the mirror-side movable electrode 85 and the mirror side fixed electrode 87a operating together, the mirror 83 connected to the mirror arm 84 is rotatable (swingable) around the rotation axis of the torsion bars 86, 86 together with the mirror arm 84 and the mirror-side movable electrode 85.
An inner frame arm 87b is provided on a side in the inner frame 87 opposite to the side where the mirror 83 is placed, and an inner-frame-side movable electrode (comb-like electrode) 87c is provided along the side surface of the inner frame arm 87b. On an outer frame arm 88a, an end of which is fixed on an outer frame 88, an outer frame fixed electrode (comb-like electrode) 88b is provided. The inner-frame-side movable electrode 87c and the outer frame fixed electrode 88b are placed opposite to each other. The inner frame 87 is supported by the outer frame 88 so as to be rotatable around torsion bars 89, 89, with one side being connected to the inner frame arm 87b. 
With the electrostatic force generated by operating the inner-frame-side movable electrode 87c and the outer-frame-side fixed electrode 88b together, the inner frame 87 is rotatable (swingable) around a rotation axis of the torsion bars 89, 89.
Each of the above-described micro-mirror elements 82 has the inner frame 87 arranged so as to cover the mirror 83, and for that reason, portions of two of the inner frames 87 are positioned between the two adjacent mirrors 83. As a result, mirror occupancy (mirror width Wm relative to mirror pitch Wp) in a mirror alignment direction cannot be increased.
For example, an optical wavelength selection switch splits an optically multiplexed signal from an input port to single signals with every wavelength, reflects these signals with a micro-mirror array, and sorts the signals into desired output ports. The micro-mirror array used here needs to have a wide reflection (transmission) band of optical signals (a narrow reflection band results in degradation of transmission quality of the optical signals). Although the mirror array is arranged in accordance with the signal (wavelength) interval of the optically multiplexed signals, the mirror reflection (transmission) band with respect to an optical signal is determined by the mirror occupancy, and therefore greater mirror occupancy is desired.
In the micro-mirror array disclosed in Patent Document 2, micro-mirror elements arranged so as to project mirrors are alternately disposed in a mirror alignment direction so as to be opposite to each other and have the mirrors close to one another in the mirror alignment direction.
Patent Document 1: Japanese Laid-open Patent Publication No. 2005-305582
Patent Document 2: U.S. Pat. No. 6,984,917